The Services still need graduate officers
Not a reason! UAS's provide the RAF with very few junior officers each year (typically 4/5 per UAS) and over half joined the UAS as bursars (i.e. were "in" before even arriving at the UAS).
The only benefit to the RAF I can see is the idea that the sort of people you currently find on a UAS are the sorts of people that, in time, may well end up as some of life's "high flyers". So a positive experience on a UAS for those people will leave a positive attitude and improved awareness of the forces among "tomorrow's leaders".
But how do you measure that? To me it's highly ambiguous and very questionable if it's worth ALL that money...
How many universities are actually covered by the current UASs?
They all are, technically. It's just some you must travel further from to attend Town Nights, the airfield, etc. And some are simply so far away that it's basically impossible.